This invention relates to a tandem master cylinder for a hydraulic brake unit with slip control. In the housing of the brake unit a primary piston preloaded by a second return spring define a primary and a secondary pressure chamber within a longitudinal bore. The pressure chambers are connected to a fluid reservoir through intake chambers and nonreturn valves and are associated with a first and a second control valve which release or shut off a second connection between the pressure chambers and the fluid reservoir depending on an actuating force.
A tandem master cylinder of this type is described, for example, in the published German patent application No. 3627000 in conjunction with a slip-controlled brake unit. The particular feature of this tandem master cylinder is that its pistons are furnished with central control valves. Brake lines associated with the two pressure chambers communicate through intake lines with nonreturn valves incorporated therein, with motor-driven pumps whose suction connections are linked by way of a suction line to the fluid reservoir. In this manner, the central control valves operate as pressure limiting valves which are controlled depending on the pedal force and which limit the hydraulic pressure built up by the two pumps by releasing hydraulic fluid connections between the pressure chambers and the fluid reservoir depending on the foot force acting on the pedal. On brake operation and also in the event of a brake pressure control action, the hydraulic fluid flows through a small number of valves and on each slip control action, the two master cylinder pistons are completely restored in order to safeguard a maximum reserve for braking in the event of a failure of the pumps. In these known brake units it is a disadvantage that the master cylinder presents a considerable overall axial length which has a negative effect particularly when mounting the brake pressure transmitted in the engine compartment of an automotive vehicle.
It is, therefore, the object of the present invention to improve a tandem master cylinder of this type by reducing its overall axial length without impairing its functional reliability.